Humans are affecting the Earth’s systems on a global scale. Industrial pollutants are accumulating in our atmosphere with the potential for long-term impact on global climate and biodiversity. This is a story we are all coming to know well. However, there is now a new twist to the unanticipated consequences of anthropogenic climate change. In the distant future, industrial pollution—a sure sign of our technological activity—could potentially be detected from hundreds of light years away. Might extraterrestrial civilizations one day discover us by our pollution? Conversely, if aliens are anything like us, might we detect the existence of extraterrestrial civilizations by their pollution?

The search for extraterrestrial intelligence, more popularly known as SETI, has traditionally focused on looking for signs of technological activity by scanning the skies for electromagnetic radiation, such as radio waves. In other words, SETI research is typically focused on attempting to intercept the conversations of extraterrestrial civilizations. Thus far, over the 50-year history of SETI, we haven’t come up with any evidence for extraterrestrial intelligence, or ETI for short. Many have noted that this negative result doesn’t mean ETI isn’t out there: With an estimated 200 billion stars in our galaxy, there is a lot of territory to cover.

However, it might also be that we aren’t looking for the right signs of technological activity. The idea of searching for electromagnetic transmissions was first popularized when radio was our primary form of communication and Earth was therefore “radio loud.” But as we’ve moved on to digital communications, we are now becoming a radio-quiet planet. This leaves only about a 100-year window within which any extraterrestrial civilizations might detect our presence by our radio emissions. While 100 years may sound like a long time, on the scale of the cosmos, it is hardly the blink of an eye. If extraterrestrial civilizations are anything like us, their technological maturation may go through a similarly brief period of intense radio transmissions before going radio silent. The galaxy could therefore be ripe with short (approximately 100-year) flashes of radio noise pollution from technological civilizations in their infancy, each quickly snuffed out by rapidly advancing technologies. The challenge of detecting this phase of technological development is that we have to catch it in the act. It is therefore worse than looking for a needle in a haystack, as we must account for the short time such a signal could be produced against the backdrop of the billion-year history of our galaxy.



Henry Lin and collaborators at the Harvard–Smithsonian Center for Astrophysics have now proposed an alternative method for doing SETI. In a recent paper they suggest we hunt for signs of ETI not by looking for noise pollution, but instead by looking for industrial pollution in the atmospheres of alien worlds. Industrial pollutants are potentially longer lasting, so we have a higher probability of finding them. We are currently pumping pollutants known as chlorofluorocarbons into our own atmosphere as a byproduct of our technological activity. CFCs are a good candidate for detecting industrial activity, as they are not produced by natural sources. Additionally, searching for industrial pollution expands the window of time that we could detect ETI. Some CFCs have a lifetime of decades while others will be detectable for thousands of years. Detection of a short-lived CFC in the atmosphere of another world could therefore indicate an industrially active civilization—much like our own, while longer-lived CFCs would indicate that a polluting civilization existed sometime within the last 10,000 years or so.

Astrobiologists are already setting their sights on discovering life beyond our solar system by detecting biosignatures (evidence of past or present life) in the atmospheres of exoplanets orbiting other stars. So looking for technosignatures (evidence of past or present technology) is not too far off. Exoplanets are being revealed in increasing numbers thanks to the Kepler Space Telescope and other “planet hunting” missions. Many are terrestrial planets that could harbor conditions favorable to life. On such worlds, the presence of molecular oxygen, combined with a reducing gas such as methane, could be a telltale sign of life (both gases are primarily a product of biological activity on Earth). While a combination of these gases would be evidence of microbial life on another world, the detection of CFCs would be interpreted as a sign of advanced life.

Lin and his collaborators think that this kind of search could be conducted in the near future. By their calculations, the James Webb Space Telescope, currently set to launch in 2018, could detect CFCs at levels 10 times that of Earth. The caveat is that this could only be done for a planet orbiting a white dwarf star. (White dwarfs result from the death of a star like our sun.) Next-generation instruments could detect such signals for planets in systems more like our own solar system.



The idea of looking for the waste products of alien civilizations is not entirely new. Closer to home, SETI researchers have previously proposed that if advanced civilizations have in fact ever visited our own solar system, we might find evidence of alien “garbage”—i.e., leftovers from mining or observation missions. A good place to look for these artifacts is the moon. Since the moon lacks the rampant geological, and biological, activity of Earth, alien artifacts would be preserved for much longer on the moon than on Earth. Any alien space trash, mining equipment, or other tech left on the moon in the distant past could therefore still be evident on its surface.

The potential to discover alien industrial pollution or space trash provides promising new directions for SETI. However, it is an open question whether such a discovery would truly be a sign of intelligent life. In the words of lead author Henry Lin, “perhaps civilizations more advanced than us, with their own SETI programs, will consider pollution as a sign of unintelligent life since it‘s not smart to contaminate your own air.” More than anything, this approach represents an important aspect of our search for extraterrestrial intelligence, which is that often our methods to search for ETI are an intimate reflection of ourselves. Hopefully we will one day enter a phase of human technological development where we will possess the insights to look for “greener” little green men.

人类正在全球范围内影响着地球的各个系统：工业污染物在大气中积累，对全球气候和生物多样性有着潜在的长期影响。这是世人所皆知的事情。然而，任何事情都有两面性。在不久的将来，见证人类科技活动的工业污染很有可能从几百光年外就能被检测到。也就是说，某一天，地外文明也许能通过我们的污染发现我们的存在。而与此相反，如果外星人有着和我们相似的文明，或许我们也能够通过它们产生的污染来检测到它们的存在？

一直以来，搜寻地外文明计划(SETI)都在扫描来自太空的电磁波，试图从中找到地外文明科技活动的标志。话剧话说，SETI一直在试图截获地外文明的对话。然而，50年过去了，我们尚未能发现任何证据表明地外文明的存在。当然，找不到并不代表没有，毕竟银河系有着两千万颗星星，总有一些会存在地外文明。

也许，我们并没有找对所谓科技活动标志。当初提出搜寻电磁信号时是因为那时无线电是我们的主要通讯方式，地球上充满了无线电信号。如今我们都使用数字信号进行通讯，地球上的无线电信号少之又少。如果地外文明通过电磁信号来搜寻我们，那就只有100年的窗口时间。100年看起来很长，但放在宇宙中，这几乎就是一眨眼。如果地外文明和我们很相似，那么它们的科技发展也大致会和我们一样，在一段时间的无线电繁荣期后进入无线电沉寂。所以无线电污染只会在银河系里存在很短的时间(大约一百年)，之后便会随着文明的成熟发展，迅速消逝。因此，在有着十亿年历史的宇宙中搜寻这种短文明阶段的标志，无异于海底捞针。

如今，哈佛大学史密森天体物理中心的Henry Lin与同事提出了另一种方法来搜寻地外文明。在最近的一份论文中，他们建议通过寻找外星大气中的工业污染，而不是寻找噪声污染，来追寻地外文明。通常，工业污染物持续更长久，更有可能被我们发现。就如同人类，在科技活动中无时无刻的向大气中排放污染物氯氟化碳。由于自然界中并不生产氟氯化碳，因此它是一个很好的工业活动检测源。另外，寻找工业污染能扩大我们搜寻地外文明的窗口期。氟氯化碳能够存在几十年，甚至几千上万年。如果在大气中检测到了短命氟氯化碳，那说明这里有着工业活动。如果检测到了长寿氟氯化碳，那么意味着一万年之内，这里存在过工业文明。

目前，天体生物学家已经开始通过检测生物信号来搜寻太阳系外生命的存在。因此，寻找“科技信号”也定将不远。如今，在开普勒太空望远镜和其它行星搜寻任务的帮助下，越来越多的地外行星被发现。其中有很多是有孕育生命条件的岩质行星。在这种情况下，如果大气中存在氧气和甲烷，那么很有可能存在生命(这两种气体是地球生物活动的主要产物)。如果有这两种气体存在，意味着有微生物存在；如果有氟氯化碳存在，意味着有高级文明存在。

Lin和同事们认为，这种搜寻方法在不久的将来便可实施。据他们计算，预定于2018年发射的詹姆斯韦伯太空望远镜可以检测到十倍于地球的氟氯化碳的存在。可惜的是只能找到围绕白矮星旋转的行星。下一代设备将能够检测类日恒星系统中的行星。

其实，利用垃圾搜寻外星人这一想法并不新鲜。最初，SETI研究人员就曾提出，如果有外星人到访过太阳系，那么我们将有可能找到一些外星垃圾，比如开采矿物或是观测任务的遗留物。而月球则是最好的搜寻地。因为不像地球，月球上没有地质和生物活动，外星人留下的物品能够保存很久。任何外星人的太空垃圾，开采设备也好，其它科技设备也好，应该都还在月球表面。

通过工业垃圾搜寻外星人为SETI指明了新的方向。不过，另一个问题是，找到了氟氯化碳真的就意味着有智慧生物么？论文第一作者Henry Lin说，“如果存在比我们更高级的文明，如果他们也有SETI项目，他们或许会将污染当作不存在智慧生物的标志，因为污染大气很不明智。”最重要的是，这种方法代表了地外文明搜寻中很重要的一方面：搜寻地外文明的方法往往是人类自己问题的一个反映。希望有一天，我们的科技能进化到，去搜寻“绿色环保”的绿色外星人。